Phase shifting of light in coherent optical communications has various uses, including phase coding of optical pulses. Phase shifting is conventionally performed by lithium niobate phase shifters. Lithium niobate phase shifters are advantageous not least because they offer high speed operation and hence are useful for applications such as 100 Gb ethernet.
However, such devices typically require several volts of electric potential and occupy relatively large substrate areas. These characteristics are disadvantageous at least in the context of integrated CMOS devices because CMOS platforms are typically limited to 3.3V and the footprints of lithium niobate devices generally exceed what is acceptable for chip-scale integration. Moreover, lithium niobate is not compatible with silicon manufacturing.
Phase-shifters have in fact been implemented in silicon with speeds approaching those of lithium niobate phase shifters. However, such devices rely on the electro-optic effect, which in silicon is too weak to shift the phase by a useful amount over a short distance. Instead, the transit time of the optical pulse through the active length of the phase shifter may reach a significant fraction of the input radiofrequency pulse period. Hence, to avoid severely limiting the bandwidth of the device, it is desirable to apply the input signal via a travelling wave electrode.
Although useful, travelling wave electrodes also suffer a disadvantage. That is, a travelling wave electrode must be terminated to match the impedance of the driving circuit. The terminating resistor (or other element) increases the power dissipation of the device.
Hence, there remains a need for phase-shifting devices that offer speeds approaching those of lithium niobate phase shifters, are compatible with CMOS chip integration, and have less power dissipation than existing devices.